The long-term goal of this project is to develop strategies to improve the treatment of breast cancer. The aromatase inhibitors we pioneered in the early phases of this grant are now proving to be effective in the clinic. We have developed a unique preclinical model in order to study the effects of aromatase inhibitors and antiestrogens in breast cancer. In this model, tumors of human estrogen receptor positive cells transfected with the aromatase gene (MCF-7Ca) are grown in athymic mice. The model has provided accurate predictions of clinical outcome. The proposed studies will continue the direction of the current grant focusing our efforts on several significant observations made during this period. Much remains to be learned about the mechanism of effects of aromatase inhibitors when used in different strategies of treatment. In this application, we propose studies to investigate mechanisms of resistance to aromatase inhibitors and how they might be reversed. Our preliminary data suggest that HER2 is increased in letrozole treated tumors and appears to be overexpressed in letrozole resistant tumor cells. The HER2 receptor is the protein product of the HER2 proto-oncogene. Phosphorylated HER2 can activate down stream signaling pathways involved in proliferation and tumor cell survival. In Specific Aim 1, we plan to determine the expression of HER2 and MAPKinase in tumors harvested from mice following long-term treatment with letrozole and after second line treatment with antiestrogens. Studies are proposed to investigate the mechanism of letrozole resistance and the role of HER2 and MAPKinase in the tumor model (Specific Aim 2). In light of clinical findings that breast cancer patients with ER+ HER2+ tumors responded significantly better to letrozole than to tamoxifen, we will test the hypothesis, in Specific Aim 3, that tamoxifen but not letrozole leads to dimerization of the ER which can be phosphorylated by MAP kinase, the downstream mediator of HER2, and resulting in transcription. In the fourth Specific Aim, we will determine whether HER2 and MAPKinase inhibitors are effective in preventing the development of or overcoming resistance to letrozole in the xenograft model. [unreadable] [unreadable]